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Organic Chemistry: Mass Spectrometry, IR, and NMR Spectroscopy

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Terms in this set (20)

  • What is the molecular ion (M+) peak in mass spectrometry?
    The molecular ion peak corresponds to the intact molecule ionized by loss of one electron, representing the molecular weight of the compound.
  • What does the M+1 peak in a mass spectrum indicate?
    The M+1 peak arises from isotopic variants, mainly due to the presence of \(^{13}C\) atoms in the molecule.
  • How can isotopes like Cl and Br be identified in mass spectra?
    Chlorine and bromine show characteristic isotope patterns with M and M+2 peaks of roughly 3:1 for Cl and 1:1 for Br due to their natural isotopic abundances.
  • What is alpha cleavage in mass spectrometry?
    Alpha cleavage is fragmentation adjacent to a heteroatom, producing two daughter ions, often a resonance-stabilized carbocation and a radical.
  • How does mass spectrometry differentiate isotopomers?
    Mass spectrometry detects isotopomers as separate peaks differing by 1 or 2 mass units, reflecting isotopic substitutions like \(^{13}C\) or \(^{37}Cl\).
  • What is the base peak in a mass spectrum?
    The base peak is the most intense peak in the spectrum, set to 100% abundance, often corresponding to the most stable fragment ion.
  • Why do alcohols often show an M-18 peak in mass spectrometry?
    Alcohols commonly lose a water molecule (H2O, 18 amu) during fragmentation, producing an M-18 peak.
  • What information does IR spectroscopy provide about molecules?
    IR spectroscopy identifies functional groups by measuring vibrational transitions of bonds, showing characteristic absorption frequencies.
  • What causes broad O-H and N-H stretching bands in IR spectra?
    Hydrogen bonding causes broadening of O-H and N-H stretching bands in IR spectra.
  • What is the typical IR absorption range for carbonyl (C=O) groups?
    Carbonyl groups absorb strongly around 1700 cm-1, with exact frequency depending on the functional group.
  • How does Hooke's law relate to IR vibrational frequencies?
    Hooke's law relates vibrational frequency to bond strength and reduced mass; stronger bonds and lighter atoms vibrate at higher frequencies.
  • What is the fingerprint region in an IR spectrum?
    The fingerprint region (below ~1500 cm-1) contains complex absorption patterns unique to each molecule, useful for identification.
  • What is nuclear spin and how does it relate to NMR spectroscopy?
    Nuclei with odd numbers of protons or neutrons have spin, which aligns with or against a magnetic field, enabling resonance absorption in NMR.
  • Why are deuterated solvents used in NMR experiments?
    Deuterated solvents contain \(^{2}H\) which is NMR inactive, preventing solvent signals from interfering with sample spectra.
  • What determines the number of signals in a 1H NMR spectrum?
    The number of signals equals the number of chemically distinct proton environments in the molecule.
  • What does the area under a 1H NMR signal represent?
    The area under a 1H NMR peak is proportional to the relative number of protons in that chemical environment.
  • What causes splitting patterns in 1H NMR spectra?
    Splitting arises from spin-spin coupling with neighboring protons, indicating the number of adjacent hydrogens.
  • What is chemical shift in NMR spectroscopy?
    Chemical shift is the resonance frequency of a nucleus relative to a standard, influenced by the electronic environment.
  • How does molecular symmetry affect the number of NMR signals?
    Symmetry can make chemically equivalent protons produce fewer signals, reducing the total number of distinct peaks.
  • What is the NMR timescale and its significance?
    The NMR timescale (~1 second) determines if dynamic processes like conformational changes are averaged or resolved in the spectrum.